Electrically motorized vehicles such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell electric vehicles (FCEV) typically use high voltages (400-700 V) in the drive system to be able to provide high power to the drive motors. The distribution system for this high voltage is electrically isolated by insulation on the electric wires from the vehicle chassis. The isolation is designed to prevent an electrical shock hazard but can fail over time or due to a crash, which creates a potential electrocution hazard.
Society of Automotive Engineers standard SAE J1766 proposes a method to measure the insulation resistance in such a vehicle in order to monitor the health of the insulation system to insure the vehicle is safe to operate. In the SAEJ1766 test method, the voltage drop (V2) between the high voltage positive terminal of the battery and the chassis and the voltage drop (V1) between the high voltage negative terminal of the battery and the chassis are first individually measured.
To measure the effective isolation resistance (R2) of the positive high voltage battery terminal to the chassis, the voltage drop (V′1) across a test resistor (R0) that is connected from the negative high voltage battery terminal (N) to the chassis is measured. This measurement is made after waiting for the voltage in the circuit to stabilize after the test resistor is connected, due to the RC time constant caused by test resistor and the suppression and parasitic capacitors in the system. The isolation resistance (R2) is then calculated according to the equation:R2=R0(1+(V2/V1))((V1−V′1)/V′1)
To measure the effective isolation resistance (R1) from the negative high voltage battery terminal to the chassis, the voltage drop (V′2) across a test resistor (R0) that is connected from the positive high voltage terminal (P) to the chassis is next measured. Again, this measurement is made after waiting for the voltage to stabilize due to the voltage decay time constants caused by the suppression and parasitic capacitors in the system. The isolation resistance (R1) is then given by:R1=R0(1+(V1/V2))((V2−V′2)/V′2)
The decay of the voltage to steady state has a very long time constant (typically in the range of 15 seconds) due to the combination of: the electromagnetic compatibility (EMC) suppression filter capacitors; the parasitic capacitance between the chassis and the high voltage positive and negative terminals; and the large resistance value that is typically used for the test resistor. Therefore, to guarantee accurate results, one must wait a significantly long period of time after connecting the test resistor before taking a voltage measurement. This is problematic if an isolation measurement is to be undertaken while the vehicle is being driven. Because the battery has finite impedance as power passes into or out of the battery, the battery voltage varies significantly as current flows dynamically into or out of the battery.
Although the SAEJ1766 method utilizes both the initial high voltage positive and high voltage negative measurements to chassis, prior to connecting the test resistor, only a single (either positive or negative to chassis) measurement is obtained after connecting the test resistor and waiting the sufficiently long period for the RC time constant to decay. Because the differential high voltage value between the positive and negative battery terminals directly influences the individual positive and negative to chassis measurements, variations in the differential voltage will be reflected in variations in the positive or negative to chassis measurements. As a result, large errors may be introduced in any one measurement using the SAEJ1766 method. For this reason, prior art methods that used the SAEJ1766 standard generally either average many samples or alternatively performed some very low frequency filtering (either in the analysis software or as hardware based filters in the circuit) to mitigate the errors caused by the method.
In addition to the delay in measurement caused by the large time constant and the sensitivity to differential voltage fluctuations, the need to average multiple measurements further reduces the performance and response time of SAEJ1766 prior art measurement systems. Although the prior art SAEJ1766 methods could avoid the averaging of measurements by restricting the measurements to periods when the propulsion system is not active, this would dramatically limit the usefulness of the prior art approach.
What is needed is a method that: is useful in measuring the degree of isolation of chassis from the electrical system; is insensitive to variations in battery voltage and is without the delays engendered by excessive filtering or multiple measurements.
The present invention addresses this need.